CS4461

CS4461 Computer Networks Final Exam Study Guide

Virtual Circuit Networks:

Virtual Circuit networks consist of

   -path between src and dst hosts
   -VC numbers, one number for each link along the path
   -Entries in the forwarding table in each router along the path

Packets carry VC numbers in the header. Each router in the VC must rewrite this header since they may differ depending on the router.

In a Virtual Circuit network the routers must maintain connection state information for the ongoing connections. Each time a new connection is started a new entry in the forwarding table needs to be created.

Virtual Circuit Phases

   -VC setup: 
       -the sending transport layer contacts the network layer
       -specifies the receiver's address
       -waits for the network to set up the VC.
       -The network layer determines the path between src and dst
           -the series of links and routers through which all packets of the VC will travel.
       -The network layer then adds an entry in the forwarding table in each router along the path.
   -Data Transfer: VC is established and data begins to flow
   -VC teardown:
       -src or dst informs network layer of its desire to terminate the VC.
       -The network layer then updates forwarding tables to delete the routes.

Datagram Networks:

Each time an end system wants to send a packet it stamps the dst address on the packet and sends it to the network. The packet is sent through a series of routers. Each router uses the dst address to forward the packet.

   -lookup the appropriate output link interface in the table
   -forward the packet out the chosen interface

Matches the prefix of the dst address and chooses output link based on that.

Longest Prefix Matching Rule

   -find the longest matching entry in the table
   -forward the packet to the link interface associated with that prefix

Problem - contiguous blocks of address space are getting smaller meaning forwarding tables get larger. No connection state information is maintained, but forwarding state information is. This happens on a specified interval of time rather than each time a connection is started and ended.

Problem - Forwarding tables can be modified before all the info is transmitted so the paths of individual packets may differ and packets may arrive out of order.